Low salinity water injection alternating with immiscible gas injection to increase oil recovery

ABSTRACT

A method for recovering oil from an earth reservoir includes injecting low salinity water into the reservoir using a low salinity water injection system, the low salinity water having a salinity that is less than connate water of the reservoir or normal water that was injected in a previous water-flooding process, the normal water having a salinity that is within plus or minus ten percent of the reservoir connate water. The method further includes injecting an immiscible gas into the reservoir using an immiscible gas injection system following the injection of the low salinity water, the immiscible gas being immiscible to the low salinity water and the oil in order to recover the oil.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of an earlier filing date from U.S. Provisional Application Ser. No. 62/089,707 filed Dec. 9, 2014, the entire disclosure of which is incorporated herein by reference.

BACKGROUND

Oil recovery processes may include injecting water into an oil-bearing reservoir in order to force oil out of the reservoir and into a borehole from which it can be recovered. This process is generally referred to as water-flooding. It has been recognized that after water-flooding is carried out more oil may still remain in the pores of the reservoir rock. Hence, it would be well received in the oil industry if enhance oil recovery processes could be developed to produce more oil out of oil-bearing reservoirs after a water-flooding process is implemented.

BRIEF SUMMARY

Disclosed is a method for recovering oil from an earth reservoir. The method includes: injecting low salinity water into the reservoir using a low salinity water injection system, the low salinity water having a salinity that is less than connate water of the reservoir or normal water that was injected in a previous water-flooding process, the normal water having a salinity that is within plus or minus ten percent of the reservoir connate water; and injecting an immiscible gas into the reservoir using an immiscible gas injection system following the injection of the low salinity water, the immiscible gas being immiscible to the low salinity water and the oil in order to recover the oil.

Also disclosed is an apparatus for recovering oil from an earth reservoir. The apparatus includes: a low salinity water injection system configured to inject low salinity water into the reservoir through a borehole penetrating the reservoir, the low salinity water having a salinity that is less than connate water of the reservoir or normal water that was injected in a previous water-flooding process, the normal water having a salinity that is within plus or minus ten percent of the reservoir connate water; an immiscible gas injection system configured to inject an immiscible gas into the reservoir through a borehole penetrating the reservoir, the immiscible gas being immiscible to the low salinity water and the oil; and a controller configured to operate the low salinity water injection system and the immiscible gas injection system such that the immiscible gas is injected after the low salinity water is injected.

BRIEF DESCRIPTION OF THE DRAWINGS

The following descriptions should not be considered limiting in any way. With reference to the accompanying drawings, like elements are numbered alike:

FIG. 1 illustrates a cross-sectional view of an embodiment of apparatus for injecting connate water, low salinity water, and immiscible gas into an oil-bearing reservoir; and

FIG. 2 is a flow chart for a method for recovering oil from an earth reservoir.

DETAILED DESCRIPTION

A detailed description of one or more embodiments of the disclosed apparatus and method presented herein by way of exemplification and not limitation with reference to the figures.

Disclosed are apparatus and method for recovering oil from an oil-bearing earth reservoir. In general, the oil is found in pores of the reservoir rock. Water-flooding of the reservoir using connate water may be used first to force oil out of the pores and into a borehole for recovery. Next, low salinity water is injected into the reservoir followed by injection of an immiscible gas to produce a three-phase mixture of water, immiscible gas and oil having a lower residual saturation thus forcing at least some remaining oil in the rock pores into the recovery borehole. This process may be followed by one or more cycles of low salinity water injection and followed by immiscible gas injection. These additional cycles force low salinity water and immiscible gas into smaller pores that were previously unaffected since the larger pores are now filled with the three-phase mixture. The result is the production of even more oil. These additional cycles may be repeated until the additional amount of oil produced diminishes according to the law of diminishing returns and it is no longer economically feasible to continue to implement the cycles.

While each of the low salinity injection mechanism and the immiscible gas injection mechanism reduce residual oil saturation (S_(or)) in the rock pores, these mechanisms when combined may have more than an additive effect. One of the main mechanisms in which low salinity water and immiscible gas injection reduces Sor is due to three-phase (i.e., three separate components and not a homogeneous mixture) relative permeability characteristics of oil, water, and gas flowing in a porous media. Immiscible gas injection causes trapped gas saturation (Sgt) and this saturation reduces Sor. Further, the more water wet the rock in the reservoir due to the low salinity water, the more the trapped gas saturation decreases the Sor.

Low salinity water injection has several mechanisms which may reduce Sor and the main mechanism may be considered as wettability alteration. Injecting low salinity water after a water flood with water that is at a much lower salinity than the connate or water flood water causes the pores of the rock to become more water wet. This increase in water wetness reduces Sor.

Low salinity water injection and immiscible gas injection combine the characteristics of each approach in a synergistic way that results in additional Sor reduction than would be achievable by each approach individually. The ability for low salinity water to cause the rock to become more water wet results on greater Sgt. Additionally, the Sgt has greater reducing effect on Sor due to three phases being present in the pore space and the effect on the Stone model empirical coefficient. Thus, low salinity water injection and immiscible gas injection can result in oil recovery that is greater than the oil recovery possible by either method itself.

The recovery mechanism theory for immiscible gas injection indicates that the four main mechanisms are oil swelling, viscosity reduction, three-phase relative permeability, and oil film flow. Oil swelling and viscosity reduction are a fluid-gas interaction whereas three-phase relative permeability and oil film flow are rock-fluid property mechanisms. In immiscible gas injection, the choice of both gas and water character affects all of these mechanisms. Additionally the wettability state of the rock affects the three-phase relative permeability, and oil film flow. The rock-fluid property mechanisms have a synergistic affect with low salinity water

Three-phase permeability theory indicates that residual oil, to water and trapped gas follows equation 1. Equation 1 demonstrates a linear relationship between S_(gt) and S_(or):

S _(or) =S _(orw) −aS _(gt)  (1)

-   -   where S_(or)=residual oil saturation, S_(orw)=residual oil         saturation to water flood, a=Stone Model empirical coefficient,         and S_(gt)=trapped gas saturation.

Thus, the reduction of oil saturation and incremental oil production from immiscible gas injection is dependent on S_(gt) and “a”. Increasing either or both of these parameters will decrease S_(or). Research indicates that that trapped gas saturation is dependent on rock wettability for a defined pore geometry. Table 1 displays the variability of Sgt, where oil wet rock varies from 0.11 to 0.30 whereas water wet rock varies from 0.29 to 0.4. Research therefore indicates that immiscible gas injection will have a tendency to produce more residual oil when the rock-fluid properties indicate a more water wet system.

TABLE 1 S_(gt) Oil Wet Conditions S_(gt) Water Wet Conditions (Fraction) (Fraction) 0.22 0.32 0.30 to 0.22 0.40 to 0.29 (weakly water wet) 0.11 40% > the oil-wet value 0.11-0.25

Rock wettability also affects the empirical coefficient of equation 1. As rock becomes more water wet, there is evidence that the magnitude of “a” increases according to Table 2. Additionally, research has shown in weakly oil wet core flood experiments that trapped gas saturation (S_(gt)) did not reduce Sor below Sorw. This indicates that “a” is approximately zero for weakly oil wet rock. Thus, immiscible gas injection is more effective and has increased recovery in reservoirs which are more water wet.

TABLE 2 Wetability “a” Coefficient Water-wet 0.5 Oil-wet 0 Water-wet 0.5 to 1.0 Water-wet 0.75 Mixed-wet 0.25 Oil-wet 0.04

Low salinity water is water which has salinity lower than the connate water in the reservoir it is being injected into and lower than the waterflood injected water. For teaching purposes low salinity water refers to water that is within +/−10% of the salinity of connate water of the reservoir and/or the salinity of the water injected during a water-flood process. Low salinity water in pores reduces water relative permeability, increases oil relative permeability, and decreases S_(orw) according to research. In addition, low salinity water induces wettability alteration such that the rock in contact with the low salinity water becomes more water wet. The theory for the increased water wet character is by ion exchange and mineral reactions with the low salinity water based on laboratory evidence.

The combined effect of low salinity water injection increasing water wetness and immiscible gas injection causing the immiscible gas to be trapped in pores is additive in the three-phase system described by equation 1. Low salinity water injection will drive “a” in equation 1 towards a value of 1, thus reducing S_(or) in the presence of the immiscible gas. Additionally, the increased water wetness of the system causes S_(gt) to obtain a larger magnitude again reducing the attainable S_(or) value. Therefore applying low salinity water injection and immiscible gas injection has a combination effect of water wetting increase and non-wetting phase saturation increase, which both reduce S_(or).

FIG. 1 illustrates apparatus for implementing the enhanced oil recovery techniques discussed above. The earth 3 includes a oil-bearing reservoir having oil disposed in pores of the reservoir rock. An oil-recovery borehole 2 and an injection borehole 5 penetrate the reservoir 4. The oil-recovery borehole 2 is configured to recover oil from the reservoir 4 that is displaced by various fluid injection processes using the injection borehole 5. The fluid injection processes include water-flooding, low salinity water injection, and immiscible gas injection. A water-flooding system 6 is configured to flood the reservoir 4 using connate water or normal water having salinity within +/−10% of the salinity of the connate water. The water-flooding system 6 includes components (now shown) such as a water supply, pumps, pipes, tubulars, sensors, and valves (manual or remote controlled) necessary to water-flood the reservoir 4. The water-flooding system 6 may be controlled by a controller 7. A low salinity water injection system 8 is configured to inject low salinity water into the reservoir 4. The low salinity water has a salinity that is less than the salinity of connate water or the normal water used for the water-flooding process. The low salinity water injection system 8 includes components (now shown) such as a low salinity water supply, pumps, pipes, tubulars, sensors, and valves (manual or remote controlled) necessary to inject the low salinity water into reservoir 4. The low salinity water injection system 8 may be controlled by a controller 9. The low salinity water supply may be implemented by a low salinity membrane system or a desalination system as non-limiting embodiments. An immiscible gas injection system 10 is configured to inject an immiscible gas into the reservoir 4. In one embodiment, the immiscible gas is nitrogen. Other gases such as CO₂, methane gas, or flue gas (from hydrocarbon combustion) may also be used as long as they are at a pressure below the minimum miscible pressure of the gas so that the gas remains immiscible with the low salinity water and the oil in the pores of the reservoir. The immiscible gas injection system 10 includes components (now shown) such as an immiscible gas supply, pumps, pipes, tubulars, sensors, and valves (manual or remote controlled) necessary to inject the immiscible gas into reservoir 4. The immiscible gas supply may be implemented by a membrane or cryogenic system for producing nitrogen from air, a CO₂ pipeline, or a methane gas pipeline as non-limiting embodiments. The immiscible gas injection system 10 may be controlled by a controller 11. An oil recovery system 12 is configured to recover oil from the oil-recovery borehole 2. The oil recovery system 12 includes pumps, pipes, tubulars, sensors, and valves (manual or remote controlled) necessary to recover oil from the reservoir 4. The oil recovery system further includes a sensor 15 configured to sense an oil flow rate or an amount of oil extracted from the borehole 2 and, thus, measure an indication of an amount of oil recovered by the oil recovery system 12. The oil recovery system 12 may be controlled by a controller 13. A supervisory controller 14, which may be implemented by a computer processing system, may be used to control and coordinate the operation of oil recovery system and the various fluid injection systems. In one or more embodiments, the supervisory controller 14 may be configured to perform the functions of the controllers 7, 9 or 11 or some combination thereof.

FIG. 2 is a flow chart for a method 20 for recovering oil from an earth reservoir. Block 21 calls for injecting low salinity water into the reservoir using a low salinity water injection system, the low salinity water having a salinity that is less than connate water of the reservoir or normal water that was injected in a previous water-flooding process, the normal water having a salinity that is within plus or minus ten percent of the reservoir connate water. Block 22 calls for injecting an immiscible gas into the reservoir using an immiscible gas injection system following the injection of the low salinity water, the immiscible gas being immiscible to the low salinity water and the oil. Block 23 calls for measuring an indication of an amount of oil recovered due to the low salinity water injection and the immiscible gas injection using a sensor. Block 24 calls for determining if the amount exceeds a threshold level. Block 25 calls for repeating the injecting low salinity water, injecting immiscible water, measuring and determining in response to the amount exceeding the threshold level. Block 26 calls for not repeating the injecting low salinity water, injecting immiscible water, measuring and determining in response to the amount not exceeding the threshold level. In one or more embodiments, the threshold level is determined by the economics of continuing to inject the low salinity water and the immiscible gas based on the price of oil. If the revenue from the extraction of oil is less than the cost to continue the injection process, then the injection process may be stopped.

The above disclosed low salinity water injection and the immiscible gas injection implement in combination provide several advantages. One advantage is that they may be used where conventional miscible gas injection at shallower depths is not feasible because the minimum miscible pressure of the gas is so high that it may exceed the parting pressure of the formation rock causing the rock to fracture. In addition, when heavier oil is at shallower depths, the minimum miscible pressure of the gas may be too high for those depths because heavier oil has higher minimum miscible pressures. Low salinity water injection and immiscible gas injection is an alternative to miscible displacement. Often circumstances are such that the injected gas e.g., N₂, CO₂, CH₄) cannot be brought to a minimum miscible pressure (MMP) with oil at reservoir conditions. This scenario is often the case in shallow light oil reservoir where the MMP would be greater that rock parting pressure. Another case would be deeper reservoirs which do not have an economic miscible gas source nearby. Further, the low salinity water injection and immiscible gas injection will reduce the size of the necessary gas volume and delivery equipment. As an example a nitrogen gas ASU (air separation unit) plant can be smaller than a continuous injection scheme thus reducing project capital costs while adding to reserve growth.

Set forth below are some embodiments of the foregoing disclosure.

Embodiment 1

A method for recovering oil from an earth reservoir, the method comprising: injecting low salinity water into the reservoir using a low salinity water injection system, the low salinity water having a salinity that is less than connate water of the reservoir or normal water that was injected in a previous water-flooding process, the normal water having a salinity that is within plus or minus ten percent of the reservoir connate water; and injecting an immiscible gas into the reservoir using an immiscible gas injection system following the injection of the low salinity water, the immiscible gas being immiscible to the low salinity water and the oil in order to recover the oil.

Embodiment 2

The method according to claim 1, further comprising recovering the oil using an oil recovery system after the injecting the immiscible gas.

Embodiment 3

The method according to claim 1, further comprising measuring an indication of an amount of oil recovered due to the low salinity water injection and the immiscible gas injection using a sensor

Embodiment 4

The method according to claim 2, further comprising determining if the amount exceeds a threshold level using a controller.

Embodiment 5

The method according to claim 4, further comprising repeating the injecting low salinity water, injecting immiscible water, measuring and determining in response to the amount exceeding the threshold level.

Embodiment 6

The method according to claim 4, further comprising not repeating the injecting low salinity water, injecting immiscible water, measuring and determining in response to the amount not exceeding the threshold level.

Embodiment 7

The method according to claim 1, wherein the immiscible gas comprises at least one of nitrogen gas, carbon dioxide gas, and methane gas.

Embodiment 8

The method according to claim 1, wherein the immiscible gas comprises flue gas from hydrocarbon combustion.

Embodiment 9

The method according to claim 1, wherein the injecting low salinity water and the immiscible gas is performed using a fluid injection borehole and the oil is recovered using an oil recovery borehole that is different from the fluid injection borehole.

Embodiment 10

An apparatus for recovering oil from an earth reservoir, the apparatus comprising: a low salinity water injection system configured to inject low salinity water into the reservoir through a borehole penetrating the reservoir, the low salinity water having a salinity that is less than connate water of the reservoir or normal water that was injected in a previous water-flooding process, the normal water having a salinity that is within plus or minus ten percent of the reservoir connate water; an immiscible gas injection system configured to inject an immiscible gas into the reservoir through a borehole penetrating the reservoir, the immiscible gas being immiscible to the low salinity water and the oil; and a controller configured to operate the low salinity water injection system and the immiscible gas injection system such that the immiscible gas is injected after the low salinity water is injected.

Embodiment 11

The apparatus according to claim 10, further comprising an oil recovery system configured to recover the oil after the injecting the immiscible gas.

Embodiment 12

The apparatus according to claim 10, further comprising a sensor configured to measure an indication of an amount of oil recovered due to the low salinity water injection and the immiscible gas injection.

Embodiment 13

The apparatus according to claim 12, wherein the controller is further configured to (a) receive the indication or an amount of oil recovered by an oil recovery system due to the injecting of the low salinity water and the immiscible gas and (b) determine if the amount exceeds a threshold level.

Embodiment 14

The apparatus according to claim 13, wherein the controller is further configured to repeat the injecting low salinity water, injecting immiscible water, measuring and determining in response to the amount exceeding the threshold level.

Embodiment 15

The apparatus according to claim 13, wherein the controller is further configured to not repeat the injecting low salinity water, injecting immiscible water, measuring and determining in response to the amount not exceeding the threshold level.

Embodiment 16

The apparatus according to claim 10, wherein the immiscible gas comprises at least one of nitrogen gas, carbon dioxide gas, and methane gas.

Embodiment 17

The apparatus according to claim 10, wherein the immiscible gas comprises flue gas from hydrocarbon combustion.

In support of the teachings herein, various analysis components may be used, including a digital and/or an analog system. For example, the controllers or sensors may include digital and/or analog systems. The system may have components such as a processor, storage media, memory, input, output, communications link (wired, wireless, optical or other), user interfaces, software programs, signal processors (digital or analog) and other such components (such as resistors, capacitors, inductors and others) to provide for operation and analyses of the apparatus and methods disclosed herein in any of several manners well-appreciated in the art. It is considered that these teachings may be, but need not be, implemented in conjunction with a set of computer executable instructions stored on a non-transitory computer readable medium, including memory (ROMs, RAMs), optical (CD-ROMs), or magnetic (disks, hard drives), or any other type that when executed causes a computer to implement the method of the present invention. These instructions may provide for equipment operation, control, data collection and analysis and other functions deemed relevant by a system designer, owner, user or other such personnel, in addition to the functions described in this disclosure.

Further, various other components may be included and called upon for providing for aspects of the teachings herein. For example, a power supply (e.g., at least one of a generator, a remote supply and a battery), cooling component, heating component, magnet, electromagnet, sensor, electrode, transmitter, receiver, transceiver, antenna, controller, optical unit, electrical unit or electromechanical unit may be included in support of the various aspects discussed herein or in support of other functions beyond this disclosure.

Elements of the embodiments have been introduced with either the articles “a” or “an.” The articles are intended to mean that there are one or more of the elements. The terms “including” and “having” and the like are intended to be inclusive such that there may be additional elements other than the elements listed. The conjunction “or” when used with a list of at least two terms is intended to mean any term or combination of terms. The term “configured” relates one or more structural limitations of a device that are required for the device to perform the function or operation for which the device is configured.

The flow diagram depicted herein is just an example. There may be many variations to this diagram or the steps (or operations) described therein without departing from the spirit of the invention. For instance, the steps may be performed in a differing order, or steps may be added, deleted or modified. All of these variations are considered a part of the claimed invention.

While one or more embodiments have been shown and described, modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustrations and not limitation.

It will be recognized that the various components or technologies may provide certain necessary or beneficial functionality or features. Accordingly, these functions and features as may be needed in support of the appended claims and variations thereof, are recognized as being inherently included as a part of the teachings herein and a part of the invention disclosed.

While the invention has been described with reference to exemplary embodiments, it will be understood that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications will be appreciated to adapt a particular instrument, situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims. 

What is claimed is:
 1. A method for recovering oil from an earth reservoir, the method comprising: injecting low salinity water into the reservoir using a low salinity water injection system, the low salinity water having a salinity that is less than connate water of the reservoir or normal water that was injected in a previous water-flooding process, the normal water having a salinity that is within plus or minus ten percent of the reservoir connate water; and injecting an immiscible gas into the reservoir using an immiscible gas injection system following the injection of the low salinity water, the immiscible gas being immiscible to the low salinity water and the oil in order to recover the oil.
 2. The method according to claim 1, further comprising recovering the oil using an oil recovery system after the injecting the immiscible gas.
 3. The method according to claim 1, further comprising measuring an indication of an amount of oil recovered due to the low salinity water injection and the immiscible gas injection using a sensor.
 4. The method according to claim 2, further comprising determining if the amount exceeds a threshold level using a controller.
 5. The method according to claim 4, further comprising repeating the injecting low salinity water, injecting immiscible water, measuring and determining in response to the amount exceeding the threshold level.
 6. The method according to claim 4, further comprising not repeating the injecting low salinity water, injecting immiscible water, measuring and determining in response to the amount not exceeding the threshold level.
 7. The method according to claim 1, wherein the immiscible gas comprises at least one of nitrogen gas, carbon dioxide gas, and methane gas.
 8. The method according to claim 1, wherein the immiscible gas comprises flue gas from hydrocarbon combustion.
 9. The method according to claim 1, wherein the injecting low salinity water and the immiscible gas is performed using a fluid injection borehole and the oil is recovered using an oil recovery borehole that is different from the fluid injection borehole.
 10. An apparatus for recovering oil from an earth reservoir, the apparatus comprising: a low salinity water injection system configured to inject low salinity water into the reservoir through a borehole penetrating the reservoir, the low salinity water having a salinity that is less than connate water of the reservoir or normal water that was injected in a previous water-flooding process, the normal water having a salinity that is within plus or minus ten percent of the reservoir connate water; an immiscible gas injection system configured to inject an immiscible gas into the reservoir through a borehole penetrating the reservoir, the immiscible gas being immiscible to the low salinity water and the oil; and a controller configured to operate the low salinity water injection system and the immiscible gas injection system such that the immiscible gas is injected after the low salinity water is injected.
 11. The apparatus according to claim 10, further comprising an oil recovery system configured to recover the oil after the injecting the immiscible gas.
 12. The apparatus according to claim 10, further comprising a sensor configured to measure an indication of an amount of oil recovered due to the low salinity water injection and the immiscible gas injection.
 13. The apparatus according to claim 12, wherein the controller is further configured to (a) receive the indication or an amount of oil recovered by an oil recovery system due to the injecting of the low salinity water and the immiscible gas and (b) determine if the amount exceeds a threshold level.
 14. The apparatus according to claim 13, wherein the controller is further configured to repeat the injecting low salinity water, injecting immiscible water, measuring and determining in response to the amount exceeding the threshold level.
 15. The apparatus according to claim 13, wherein the controller is further configured to not repeat the injecting low salinity water, injecting immiscible water, measuring and determining in response to the amount not exceeding the threshold level.
 16. The apparatus according to claim 10, wherein the immiscible gas comprises at least one of nitrogen gas, carbon dioxide gas, and methane gas.
 17. The apparatus according to claim 10, wherein the immiscible gas comprises flue gas from hydrocarbon combustion. 